Cellulases or cellulolytic enzymes are enzymes involved in hydrolysis of cellulose. Typically there are three major types of cellulase enzymes involved in the hydrolysis of native cellulose, namely cellobiohydrolase (1,4-beta-D-glucan cellobiohydrolase, EC 3.2.1.91), endo-beta-1,4-glucanase (endo-1,4-beta-D-glucan 4-glucanohydrolase, EC 3.2.1.4) and beta-glucosidase (EC 3.2.1.21).
The endo-beta-1,4-glucanases (EC No. 3.2.1.4) constitute a group of hydrolases of particular interest in various industrial applications. Endoglucanases catalyse endo hydrolysis of 1,4-beta-D-glycosidic linkages in cellulose, cellulose derivatives (such as carboxy methyl cellulose and hydroxy ethyl cellulose), lichenin, beta-1,4 bonds in mixed beta-1,3 glucans such as cereal beta-D-glucans or xyloglucans, and other plant material containing cellulosic components
Cellulases are synthesized by a large number of microorganisms that include, for example, fungi, actinomycetes, myxobacteria and true bacteria, and certain plants. Endoglucanases of a wide variety of specificities have been identified.
One industrial use of cellulolytic enzymes is for treatment of cellulosic textiles or fabrics, e.g., as ingredients in detergent compositions or fabric softener compositions, for bio-polishing of new fabrics (garment finishing), and for obtaining a “stone-washed” look of cellulose-containing fabrics, especially denim. Another important industrial use of cellulolytic enzymes is the use for treatment of paper pulp, e.g., for improving the drainage or for deinking of recycled paper.
Bioconversion of renewable lignocellulosic biomass to a fermentable sugar that is subsequently fermented to produce alcohol (e.g., ethanol) or other fuels as an alternative to petroleum-based fuels is also an important use of cellulosic enzymes. Fermentable sugars are also used to produce plastics, polymers and other bio-based products and this industry is expected to grow substantially increasing the demand for abundant low cost fermentable sugars that can be used as a feed stock in lieu of petroleum based feedstocks. Cellulosic biomass is the most abundant renewable natural resource. Generated at a rate of ˜100 billion dry tons/year by the biosphere, cellulosic biomass has the potential to replace the world's demand for diminishing fossil fuels.
The major polysaccharides comprising different lignocellulosic residues, that may be considered as a potential renewable feedstock, include, for example, cellulose and hemicelluloses (xylans). The enzymatic hydrolysis of these polysaccharides to soluble sugars, for example glucose, xylose, arabinose, galactose, mannose, and other hexoses and pentoses can occur under the action of different enzymes acting in concert. Endo-1,4-β-glucanases (EG) and exo-cellobiohydrolases (CBH) catalyze the hydrolysis of insoluble cellulose to cellooligosaccharides (cellobiose as a main product), while β-glucosidases (BGL) convert the oligosaccharides to glucose. Xylanases together with other accessory enzymes (non-limiting examples of which include α-L-arabinofuranosidases, feruloyl and acetylxylan esterases, glucuronidases, and (β-xylosidases) catalyze the hydrolysis of hemicelluloses.
Regardless of the type of cellulosic feedstock, the cost and hydrolytic efficiency of enzymes are major factors that restrict the commercialization of the biomass bioconversion processes. According to Zhang, Y. H. P., “[o]ne of the most important and difficult technological challenges is to overcome the recalcitrance of natural lignocellulosic materials, which must be enzymatically hydrolyzed to produce fermentable sugars” (see, Zhang et al. (2006) Biotechnol. Adv. 24: 452-481). A major limitation for the conversion of biomass to biofuel and renewable chemicals is the high cost and large quantities of enzymes required for hydrolysis.